Objectives: Bacterial heat shock proteins (HSPs) have been implicated in the pathogenesis of several diseases, and a relationship between rheumatoid arthritis (RA) and Escherichia coli HSP40 (DnaJ) has been reported. Since the processes involved in tissue destruction in periodontitis and RA are similar, we examined the reactivities of antibodies in sera from RA patients to Actinobacillus actinomycetemcomitans DnaJ protein. Methods: We previously reported that IgG titers to the conservative N-terminal region of DnaJ were significantly higher in RA patients. Therefore, we further analyzed the immunodominant region of this N-terminus in patients with RA. N-terminal protein fragments of A. actinomycetemcomitans DnaJ were purified by affinity chromatography, and the serum IgG levels of these protein fragments were analyzed by enzyme-linked immunosorbent assay (ELISA). Results: ELISA showed that IgG titers to the fragment involved in the J-domain, the most conservative region of DnaJ, were significantly higher in RA patients than in healthy controls (p < 0.05). Furthermore, to identify the B-cell epitopes of this region, four overlapping synthetic peptides (21 amino acids) covering the entire J-domain were synthesized. Using these peptides, we analyzed the IgG titers to these peptides by ELISA, and identified the immunodominant region of A. actinomycetemcomitans DnaJ in patients with RA. Conclusions: The results suggest that the region involved in the "J-domain" of A. actinomycetemcomitans DnaJ is relevant to the abnormal immune responses in patients with RA. This investigation was supported by a Grant-in-Aid for the Encouragement of Young Scientists (No. 13771265) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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